More surprises with the `==` operator

help
21

This actually does not fail. I think I mixed something up. I'm sorry. I had a previous version that looked different when it was failing. Maybe I'll figure out again what confused me.

Okay, the surprising thing is this:

fn main() {
    let array: [u8; 1] = [0u8; 1];
    let array_ref: &[u8; 1] = &[0u8; 1];
    let slice: &[u8] = &vec![0u8];
    assert!(array == slice); // works
    assert!(array == array_ref); // fails
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0277]: can't compare `[u8; 1]` with `&[u8; 1]`
 --> src/main.rs:6:19
  |
6 |     assert!(array == array_ref); // fails
  |                   ^^ no implementation for `[u8; 1] == &[u8; 1]`
  |
  = help: the trait `PartialEq<&[u8; 1]>` is not implemented for `[u8; 1]`
  = help: the following other types implement trait `PartialEq<Rhs>`:
            <&[B] as PartialEq<[A; N]>>
            <&[T] as PartialEq<Vec<U, A>>>
            <&mut [B] as PartialEq<[A; N]>>
            <&mut [T] as PartialEq<Vec<U, A>>>
            <[A; N] as PartialEq<&[B]>>
            <[A; N] as PartialEq<&mut [B]>>
            <[A; N] as PartialEq<[B; N]>>
            <[A; N] as PartialEq<[B]>>
          and 3 others

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to previous error

I would expect that if I can't compare [0; 1] with &[0; 1], then I would also not be able to compare [0; 1] with a slice. But comparison with &[u8] works (but not with &[u8; 1]).

That is a bit... strange (but may be just like how the std library works).

It works exactly because of this impl:

So unsized coercion is indeed not involved.

Side question: How can it be existing since Rust 1.0 while it uses const generics? (Might need to look that up in the repository history.)